improving ceiling isolation in lower office

[jasman]

Hi folks,
First post, long time occasional looker.

I just bought a house with a 600 sq. ft. office on a lower level, underneath the main house's dining room and bedroom. I want to beef up the ceiling isolation so that it can be used as a quiet office for treatments and therapy sessions. That means reducing footfalls and conversation from above from tenants that rent the main house. This is not exactly a studio problem, but similar. I am a musician with a separate studio in another house, and have some acoustical design experience, so that's why I'm posting here. The floor above is fir planking over a plywood subfloor. I can put rugs with padding up there if need be to help with footfalls.

I plan on removing the sheetrock from the ceiling, adding more sheetrock mass to the underside of the floor, in the usual way, then install a new ceiling with 2 layers of 5/8" gypsum board on RC. I think. Here's what I don't have a good sense of: There is one wooden beam that runs the center of the room, supported by one lally column. It stands proud of the existing finished ceiling by about 3". Because of that I assume the ceiling joists are probably hung with joist hangers on either side of it. The question is how much sound transmission, roughly, would you expect to be transmitted through the beam and column, if the rest of the ceiling was treated as described? Or is it silly to do that treatment to the rest of the ceiling if the beam will be a major weak link? I don't want to drop the finished ceiling height enough to be below the beam level, but I could surround the beam with a deadening soffit of some sort. I'm not planning to do anything to the walls. Two of them are more or less under ground and two are exposed to air, but no other sound sources from the house other than what is transmitted to them from above.

My sense is that because the beam and column are under load, they won't be flexing much in response to sound, but I may be underestimating that.

Any helpful suggestions or shared experience would be appreciated.
James

[xSpace]

"My sense is that because the beam and column are under load, they won't be flexing much in response to sound, but I may be underestimating that."

Classical flanking path...

[Ted White]

It is a flanking path, but you can still proceed.

Add the 5/8" drywall to the underside of the floor as you already mentioned. The more the merrier. Use a damping compound for maximum damping effect.

Then install simple R19 fiberglass.

Strongly consider using a competent clip & channel system rather than resilient channel (RC-1). RC-1 has many issues.

Finish the ceiling with double layers of standard 5/8" drywall. Again, damp these drywall panels.

There is more detail and diagrams here: http://www.soundproofingcompany.com/sol ... _ceilings/

[jasman]

Thank you Ted, that's helpful. A couple more areas of uncertainty for me:
1. Do you have a sense of how far to go with the extra mass and new layers of sheetrock, before it won't make sense due to the overall performance being limited by the exposed beam?
2. If I do leave the beam exposed, how would you suggest detailing the intersection of new sheetrock and the protruding beam? Caulk and trim strip to hide, or what?
3. Related to that, how to detail the intersection of the ceiling and wall sheetrock. Would a tape and mud corner, over caulk, compromise the performance.
4. I've looked over this forum and have yet to understand if screwing the sheetrock to the underside of the subfloor is problematic. I do get that it needs to be caulked at the edges. But if it is floating in the middle and secured by edge strips to the joists, is that somehow better than screwing it directly to the subfloor?
5. I'm still not clear how to attached the sheetrock if using GG in between layers.
Thanks much,
James

[Ted White]

Hi James,

While opinions may vary, here's mine:

1) No real sense as all instances are different. Generally the method I mentioned previously is quite dramatic, and I would not suggest more.

2) You would essentially caulk and trim, yes. You would like as lossy a connection at that point as possible, without having a large gap that may create a seal failure at some point.

3) Same thing here. Caulk to seal, then mud and tape.

4) If you use a damping compound with the drywall on the underside of the floor, you would use standard drywall screws to compress the compound to a thin film.

5) Just screw through the drywall or plywood layers to compress the Glue.

[Cram]

I'm curious -
Why are you not willing to wrap or isolate the beam? You could use the same clip or rc system you suggest for the ceiling, but you'd simply be framing the beam in and gaining isolation.

In my room, I had a similar beam with a column. I wrapped it like I mention, but I used the two walls to carry the span to wrap the beam and column with drywall. Nothing touches the beam or drywall at all.

It seems you're not concerned with too much low frequency coming out, but foot traffic coming down, so perhaps your answer to the question is based in cost and efficiency of isolation benefit.

all the best!

[xSpace]

Thank you Ted, that's helpful. A couple more areas of uncertainty for me:
1. Do you have a sense of how far to go with the extra mass and new layers of sheetrock, before it won't make sense due to the overall performance being limited by the exposed beam?

Overall performance isn't the contributing factor. It is dead load/how much weight can your as YET dimensioned overhead floor actually take.

You should present the details about the measurements of the room and the dimensions of the lumber involved before someone gets you into a difficult situation with adding too much weight to an area that no one has really been able to determine if this is a good idea or not.

[xSpace]

The more the merrier.

I doubt it...but you preach on brother Ted:)

[gullfo]

and ideally the soffit you create for the beam has the same mass of the rest of the room and if (assuming the framing it attached to the existing structure and not the new isolation walls) then using isolation clips and hat channel to decouple the drywall.

[Ted White]

Good point. And have a bit of standard thermal fiberglass uncompressed in the joists and in that soffit.